2019-040 - Mainplace Mall Transformation

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Water Quality Management Plan (WQMP) Mainplace Mall Specific Plan The TGD includes the following caveat, "This TGD... interprets this provision such that increases in Tc would be acceptable and reduction in Tc of more than 5 percent would not be acceptable. This interpretation is consistent with the overall goal of the permit to protect receiving waters from stormwater impacts to the maximum extent practicable." Since the various parcels on the site are fully developed in the existing condition, no substantial changes to the 2-year storm volumes or times of concentration are anticipated; therefore, the specific plan projects are not expected to cause erosion or siltation in the downstream receiving waters. Threshold (e) Would the project create or contribute runoff water which would exceed the capacity of existing or planned storm water drainage systems or provide substantial additional sources of polluted runoff? (CEQA Environmental Checklist: Hydrology and Water Quality) Individual projects in the Specific Plan, as developed, will be required to implement Beast Management Practices (BMPs) to reduce the pollutants present in developed site stormwater to the Maximum Extent Practicable (MEP). Individual WQMPs specific to each priority project moving forward will be required. Each of those WQMPs will outline the site -specific BMPs required to achieve the MEP standard. The Projects in the specific plan are expected to produce several storm water pollutants including sediments, Nutrients, heavy metal, bacteria and other pathogens, oil and grease toxic organic compounds, and oil and grease. Reach 2 of the Santa Ana River, one of the site's receiving waters, is listed as impaired for indicator bacteria. Projects within the specific plan will include BMPs that reduce the levels of bacteria in site runoff and reduce the levels of other expected stormwater pollutants. The projects within the Specific Plan will utilize Low Impact Development (LID) BMPs to reduce the stormwater contaminants that leave the proposed developments, as prescribed by the TGD and City of Santa Ana. Per the County's Model Water Quality Management Plan (MWQME), an LID BMP "provides retention or biotreatment as part of an LID strategy - these may include hydrologic source controls (HSCs), retention, and biotreatment BMPs." The MWQMP set forth the following LID design criteria: • Priority Projects must infiltrate, harvest and use, evapotranspire, or biotreat/biofilter, the 85th percentile, 24-hour storm event (Design Capture Volume). • A properly designed biotreatment system may only be considered if infiltration, harvest and use, and evapotranspiration (ET) cannot be feasibly implemented for the full design capture volume. In this case, infiltration, harvest and use, and ET practices must be implemented to the greatest extent feasible and biotreatment may be provided for the remaining design capture volume. Centennial Section II Conceptual Specific Plan WQMP Page 10